1. Field of the Invention
This invention relates to modulators, especially to modulators for applying digital modulation to generate modulated data outputs, for example, for use in transceivers (transmitters/receivers) for use with a local area network.
2. Description of the Prior Art
Modulators employing a voltage controlled oscillator in a phase locked loop are used for producing a frequency modulated output e.g. of analogue signals such as speech. Such modulators are also employed for modulating digital data.
For example, such a modulator is shown in FIG. 1. In the absence of a modulating input applied to modulation means 1 i.e. if a continuous input of digital state 0 or digital state 1 is applied, the output of the voltage controlled oscillator 2 is held to N.F.sub.ref, where N is the ratio of a divider 3 and F.sub.ref is a reference frequency produced by a crystal oscillator. Any difference between the output and N.F.sub.ref results in an error signal from a phase detector 4 which is fed as an input to the voltage controlled oscillator in a sense to reduce that difference. A low pass filter 5 removes noise generated at the voltage controlled oscillator, while allowing desired modulation frequencies (outside the bandwidth of the low pass filter 5) to modulate the output. The pass band should be as wide as possible to remove as much noise as possible, yet not so wide as to remove any low frequency component of the digital modulating signal which, in the case of some digital signals e.g. N.R.Z. (non-return to zero) can be considerable.
A problem with such a modulator arises in the case of a transceiver. Referring to FIG. 2, when the data consists of random 0 and 1 states the loop constrains the average frequency to be N.F.sub.ref (shown dotted). During a period of reception, assuming that modulating input remains (say) at digital state 0, the modulating output drifts over a period of time, from the frequency corresponding to digital state 0 to the frequency N.F.sub.ref. The output of course is not transmitted by the transceiver. The drift is faster the greater the loop bandwidth. The problem arises when transmission recommences (FIG. 4). When the data logic state 1 is applied, the instantaneous output frequency will shift by the total modulation bandwidth and will thus exceed the frequency value corresponding to digital state 1. It will then drift back to frequency values centred about N.F.sub.ref. The same applies if logic state 1 was applied during receive periods, and retransmission commences with data logic state 0.